JERUSALEM/BEIRUT, Feb 10 (Reuters) - Israel launched heavy air strikes in Syria on Saturday, saying it hit air defenses and Iranian targets, and the Syrian army claimed to have brought down an Israeli F-16 that crashed in northern Israel in a major escalation of tension.

The Israeli military said early assessments indicated the jet had been shot down by Syrian fire, but this was still unconfirmed.

It marked the most serious confrontation yet in Syria between Israel and Iranian and Iran-backed forces that have established a major foothold in the country while fighting in support of President Bashar al-Assad in the civil war.

Israel said the F-16 crashed during a mission to strike Iranian drone installations in Syria. It said it sent its jets into Syria after shooting down an Iranian drone over Israeli territory earlier on Saturday.

The military alliance fighting in support of Assad denied any of its drones had entered Israeli air space. In a statement, it said Israel had targeted an air base in the Homs desert that is being used to fly drones in missions against Islamic State.

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Struggling to survive amid shortages in Syria

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Struggling to survive amid shortages in Syria

Five-month-old twins Farah and Marah wait for their parents to receive milk distributions from a medical center in the Hamoria area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

Two-and-a-half year old Hala al-Nufi, who suffers from a metabolic disorder which is worsening due to the siege and food shortages in the eastern Ghouta, is held by her uncle in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

A pot containing food is seen in the Hazzeh area in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

Two-and-a-half year old Hala al-Nufi, who suffers from a metabolic disorder which is worsening due to the siege and food shortages in the eastern Ghouta, is held by her uncle in the Saqba area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY.

Six-month-old twins Safa and Marwa, who suffer from malnutrition, are seen at their home in the Hazzeh area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

Two-and-a-half year old Hala al-Nufi, who suffers from a metabolic disorder which is worsening due to the siege and food shortages in the eastern Ghouta, reacts as she sits on a bed in the Saqba area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY.

Six-month-old twins Safa and Marwa, who suffer from malnutrition, are held by their mother in the Hazzeh area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

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Such "terrorist action" by Israel would be met with a "severe and serious response," it said.

The Israeli military spokesman said Israel did not seek escalation in the region, calling its action a "defensive effort triggered by an Iranian act of aggression."

Iran's expanding clout during Syria's nearly seven-year-long war, including deployments of Iran-backed forces near the Golan frontier, has raised alarm in Israel, which has said it would act against any threat from its regional arch-enemy Tehran.

Iranian and Iran-backed Shi'ite forces, including Lebanon's Hezbollah, have deployed widely in support of Assad. Iran's military chief warned Israel last October against breaching Syrian airspace and territory.

Israel's air force has targeted Syrian military and Hezbollah targets in Syria on an almost regular basis, but its attacks on Saturday appeared to be the most intense yet.

Referring to the downed Israeli F-16, an official in the pro-Assad alliance said a "message" had been delivered to Israel. "I do not believe matters will develop to a regional war," the official said.

U.S. HAWKISH ON IRAN

In Washington, President Donald Trump’s administration has backed Israel’s hawkish stance on Iran, and declared containing Tehran's influence an objective of its Syria policy. On a visit to Israel last month, U.S. Vice President Mike Pence called Iran the world’s "leading state sponsor of terror."

U.S. Secretary of State Rex Tillerson is also expected to visit the region in the coming week to discuss the crisis in Syria and other issues, and is scheduled to visit Jordan, Turkey, Lebanon and other countries.

Hezbollah and Israel last fought a major conflict in 2006.

Tensions have also spiked across the frontier between Israel and Lebanon over Israeli plans for border wall, and Lebanese plans to exploit an offshore energy block which is partly located in disputed waters.

The Israeli military said 12 targets, including three aerial defense batteries and four Iranian targets that are part of Iran's military establishment in Syria were attacked.

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Heartbreaking scenes from Eastern Ghouta, Syria

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Heartbreaking scenes from Eastern Ghouta, Syria

Six-month-old twins Safa and Marwa, who suffer from malnutrition, are seen at their home in the Hazzeh area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

Two-and-a-half year old Hala al-Nufi, who suffers from a metabolic disorder which is worsening due to the siege and food shortages in the eastern Ghouta, reacts as she sits on a bed in the Saqba area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY.

A Syrian shovels away debris from the higher floor of a building that was reportedly shelled by regime forces in the rebel-held Eastern Ghouta town of Kafr Batna on the outskirts of Damascus on November 20, 2017. / AFP PHOTO / ABDULMONAM EASSA (Photo credit should read ABDULMONAM EASSA/AFP/Getty Images)

ATTENTION EDITORS - VISUALS COVERAGE OF SCENES OF DEATH AND INJURY An injured student lies on a bed at a field hospital in the town of Jisreen, in the eastern Damascus suburb of Ghouta, Syria October 31, 2017. REUTERS/Bassam Khabieh TEMPLATE OUT

DAMASCUS, SYRIA - NOVEMBER 20: People inspect the debris of a building after Assad regime's warcrafts carried out airstrikes over residential areas of Kafr Batna town of the Eastern Ghouta region of Damascus in Syria on November 20, 2017. It is reported that 9 civilians, including 4 children were killed after the attack.
(Photo by Anas Damashqy/Anadolu Agency/Getty Images)

ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH A wounded man is seen lying in Douma hospital after heavy shelling in the rebel-held besieged town of Douma, eastern Ghouta in Damascus, Syria, November 19, 2017. REUTERS/Bassam Khabieh TEMPLATE OUT

ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH A wounded man is seen lying in Douma hospital after heavy shelling in the rebel-held besieged town of Douma, eastern Ghouta in Damascus, Syria, November 19, 2017. REUTERS/Bassam Khabieh

A man walks on rubble after an airstrike in the rebel-held city of Douma, in the eastern Damascus suburb of Ghouta Syria November 2, 2017. REUTERS/Bassam Khabieh

Two-and-a-half year old Hala al-Nufi, who suffers from a metabolic disorder which is worsening due to the siege and food shortages in the eastern Ghouta, is held by her uncle in the Saqba area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY.

Children wait to be examined from Unicef health workers in Kafra Batna in the Damascus suburb of eastern Ghouta, Syria October 30, 2017. REUTERS/Bassam Khabieh

ATTENTION EDITORS - VISUALS COVERAGE OF SCENES OF DEATH AND INJURY An injured student lies on a bed at a field hospital in the town of Jisreen, in the eastern Damascus suburb of Ghouta, Syria October 31, 2017. REUTERS/Bassam Khabieh TEMPLATE OUT

ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH Injured students lie on beds at a field hospital in the town of Jisreen, in the eastern Damascus suburb of Ghouta, Syria October 31, 2017. REUTERS/Bassam Khabieh

Smoke rises at a damaged site in Ain Tarma, eastern Damascus suburb of Ghouta, Syria, September 14, 2017. REUTERS/Bassam Khabieh

"During the attack, anti-aircraft missiles were fired towards Israel, triggering alarms that were heard in Northern Israel," the military said.

Syrian state media reported two separate Israeli attacks.

In the first one, a military source said Syrian air defenses had opened fire in response to an Israeli act of "aggression" against a military base, hitting "more than one plane."

Later, state media said air defenses were responding to a new Israeli assault and air defenses had thwarted attacks on military positions in southern Syria.

Israel said one of its attack helicopters shot down an Iranian drone at around 4.30am (0230 GMT) that had come from Syria into Israel. "In response, the IDF (Israel Defence Forces) targeted Iranian targets in Syria," the military said.

"MASSIVE" ANTI-AIRCRAFT FIRE

Israeli military spokesman Jonathan Conricus said a "substantial" number of Israeli warplanes on the mission had come under "massive Syrian anti-air fire," and only one Israeli jet was harmed.

The F-16 came down in a field near the northern Israeli village of Harduf, television footage showed, and one of the pilots was injured as they ejected, the military said.

David Ivry, a former Israeli Air Force chief, told Reuters he believed it was the first time an Israeli F-16 was brought down since Israel began using the jets in the 1980s.

"We don't know if the pilots ejected because of the (Syrian)fire," Conricus said. It was also unclear at what stage of the mission they ejected, he said, "but it is of extreme concern to us if they were shot down."

15PHOTOS

Moments of childhood fun in war-torn Syria

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Moments of childhood fun in war-torn Syria

Children play with water from a burst water pipe at a site hit yesterday by an air strike in Aleppo's rebel-controlled al-Mashad neighbourhood, Syria, September 30, 2016. REUTERS/Abdalrhman Ismail

Children ride in carts on the third day of Eid al-Adha in the rebel controlled city of Idlib, Syria September 14, 2016. REUTERS/Ammar Abdullah

A boy inflates a balloon along a street on the first day of the Muslim holiday of Eid al-Fitr, which marks the end of the holy month of Ramadan, in the rebel held Douma neighborhood of Damascus, Syria July 6, 2016. REUTERS/Bassam Khabieh.

Youths play soccer on a street in Manbij, in Aleppo Governorate, Syria, August 9, 2016. REUTERS/Rodi Said

A boy carries fish in a bag along a street in the rebel held Douma neighborhood of Damascus, Syria March 26, 2016. Picture taken March 26, 2016. REUTERS/Bassam Khabieh

Boys swim to cool down from heat inside a swimming pool, in the rebel held besieged town of Douma, eastern Damascus suburb of Ghouta, Syria, June 23, 2016. REUTERS/Bassam Khabieh

A boy sits on a bicycle in front of damaged shops after an airstrike on the rebel held al-Qaterji neighbourhood of Aleppo, Syria September 25, 2016. REUTERS/Abdalrhman Ismail

Boys play with toy guns on the first day of the Muslim holiday of Eid al-Fitr, which marks the end of the holy month of Ramadan, in a rebel-held area of Aleppo, Syria, July 17, 2015. REUTERS/Abdalrhman Ismail

Boys on bicycles gather in the rebel held area of Aleppo's al-Shaar district, Syria, June 27, 2016. Picture taken June 27, 2016. REUTERS/Abdalrhman Ismail

A girl pours water over a dog to cool it down, in the rebel held besieged town of Douma, eastern Damascus suburb of Ghouta, Syria, June 23, 2016. REUTERS/Bassam Khabieh

Students form a circle as they play during a celebration marking the end of the school year, at 'Syria, The Hope' school on the outskirts of the rebel-controlled area of Maaret al-Numan town, in Idlib province, Syria June 1, 2016. The school is partially occupied and it teaches students until fourth grade. The building that is heavily damaged was used by government forces as a base before the rebel fighters took control of the area. REUTERS/Khalil Ashawi

Children play in the Douma neighborhood of Damascus, Syria November 26, 2015. REUTERS/Bassam Khabieh

A boy rides on a tricycle along a damaged street in the besieged area of Homs, January 1, 2014. Picture taken January 1, 2014. REUTERS/Yazan Homsy

Children play table football on the last day of Eid al-Adha celebrations in the rebel held besieged town of Hamouriyeh, eastern Ghouta, near Damascus, Syria September 15, 2016. REUTERS/Bassam Khabieh

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The Syrian Observatory for Human Rights said Israel had targeted areas in the countryside southwest of Damascus, near the Syrian-Lebanon border west of Damascus and in the eastern countryside of Homs province for several hours since dawn.

It said another set of raids hit southwest of Damascus, and another struck around the Damascus-Beirut highway near the border with Lebanon.

In Israel, uniformed military personnel could be seen gathered around the burnt and tangled metal in Harduf by mid-morning, with what appeared to be white foam on the surrounding grass. Others knelt in the grass, inspecting pieces of the jet.

Rocket alert sirens sounded in the Israeli-held Golan Heights and in northern Israel. There were no reports of casualties.

Flights in to Israel's main airport near Tel Aviv were suspended for about 15 minutes and take-offs were held for about 20 minutes on Saturday morning. "Ben Gurion Airport is now operating as usual," Israeli Airport Authority spokesman Ofer Lefler said.

The airport's online live flight schedule showed flights were departing and landing.